1. Field of the Invention
The present invention relates to an electric power steering device which assists steering wheel torque with a multiphase AC motor.
2. Description of the Related Arts
An electric power steering device causes an electric motor to generate auxiliary torque in accordance with steering wheel torque by a steering operation by a driver and reduces the steering wheel torque.
It is requisite for the electric power steering device to perform a control to the electric motor which does not affect the steering feeling to the driver.
PWM control by a triangular-wave comparison technique is for comparing a three-phase sine-wave command voltage with a reference triangular-wave voltage and for generating a PWM-control signal voltage. A PWM inverter generates, based on the PWM-control signal voltage, a rectangular-wave drive voltage having undergone the PWM control, applies the rectangular-wave drive voltage to the electric motor, and allows a three-phase current to flow.
Moreover, when a PWM control is performed using a PWM-control signal voltage generated with the amplitude ratio of the three-phase sine-wave command voltage being increased, the amplitude ratio is limited to a predetermined amplitude ratio because the average voltage of a PWM drive voltage fluctuates and the torque ripple of the electric motor increases. The amplitude ratio is, however, the ratio of the amplitude of a signal voltage relative to the amplitude of the reference triangular-wave voltage.
Non-patent document 1 discloses a technology of increasing the rate of utilization of voltage. According to such technology, a three-phase modulation-wave voltage that is a three-phase sine-wave command voltage modulated by a third-order integral-multiple harmonic (a triangular wave) is used for the PWM control.
The three-phase modulation-wave voltage becomes small in the vicinity of the amplitude of the signal voltage in comparison with the three-phase sine-wave command voltage. As a result, the average voltage of the PWM drive voltage does not fluctuate, and a range where the amplitude ratio can be set large expands, thereby improving the rate of utilization of voltage.
Non-Patent document 1: Theory of AC servo system and designing thereof in practice, chapter three, power converter circuit pages 44 to 45 (Jul. 10, 2005, seventh edition, published by: SOUGOU DENSHI SHUPPAN)
The technology disclosed in Non-patent document 1 improves the rate of utilization of voltage and increases a current flowing through the electric motor (multiphase AC motor), so that the electric motor can be driven with high output.
An electric power steering device to which the technology disclosed in Non-patent document 1 is applied does not affect the steering feeling to a driver even if the resolution of steering becomes small at the time of low-speed running (at the time of static steering).
However, as it is requisite to perform steering at a high resolution at the time of fast-speed running, the electric power steering device to which the technology disclosed in Non-patent document 1 is applied affects the steering feeling to the driver at the time of fast-speed running.